A magnetoresistive random access memory (MRAM), which is one of non-volatile memory devices, includes a plurality of magnetic memory cells. It is known that the magnetoresistive effect appears in multi-layer films that are alternately stacked by magnetic layers and non-magnetic layers. Magnetic resistance over a magnetic memory cell indicates minimum and maximum values when magnetic vectors in magnetic layers point in same and opposite directions, respectively. The same and opposite directions of magnetic vectors in two magnetic layers are called "Parallel" and "Antiparallel" states, respectively. When magnetic material is employed for a memory device, parallel and antiparallel directions, for example, are logically defined as "0" and "1" states, respectively.
The MRAM device normally arranges magnetic memory cells on intersections of word and sense lines, which are placed in rows and columns. The MRAM circuit, for instance, is described in U.S. patent application Ser. No. 09/055,731 entitled "MAGNETORESISTIVE RANDOM ACCESS MEMORY DEVICE AND OPERATING METHOD THEREOF," filed Apr. 1, 1998 assigned to the same assignee.
Activation of word and sense lines enables the MRAM device to access the memory cell. The sense line is directly coupled to the memory cells and a sense current flows in the magnetic layers so that a sense current is affected by magnetic vectors in the magnetic layers and the sense current value in the memory cell or the voltage drop across the memory cell is alternated according to the direction of magnetic vectors. Sensing the changes in the sense current value or the voltage drop allows one to detect states stored in the memory cells. On the other hand, a writing process is carried out by applying a sufficient magnetic field to switch magnetic vectors in the magnetic layers. In order to meet the magnetic requirements, a torque or digit line is placed in parallel with the word line to provide a digit current. The digit, word, and sense currents all create a total magnetic field and apply it to the memory cell, which stores states in the memory cell in accordance with directions of the total magnetic field.
In order to ensure a sufficient current, digit, word, and sense lines typically employ metal material, which causes a memory cell size to increase for accommodation of all three metal spacing and pitches. Accordingly, a word line is replaced with poly-silicon material to alleviate a space limitation. Replacement to poly-silicon allows an MRAM device to highly integrate memory cells. A poly-silicon line, however, has an increased resistance that causes a transmission delay of a signal on the word line and requires much access time. Therefore, the more memory cells that are integrated, the more access time is needed.
Accordingly, it is a purpose of the present invention to provide an improved MRAM device that has a high-speed operation.
It is another purpose of the present invention to provide an improved MRAM device that has a high density memory cell arrangement.
It is still another purpose of the present invention to provide an improved MRAM device that attains reduction in memory cell size.